Various motorized systems in use today include an electric motor powered by a battery or other source of electrical energy. The speed of the electric motor is commonly controlled by supplying power from the power source to the motor through a serially connected solid-state switching element. The switching element can be, for example, a transistor or silicon controlled rectifier. The switching element is typically pulse controlled by a signal chopper of conventional design.
In such systems, various circuit elements are commonly connected in parallel with the motor. A short circuit condition in one of these circuit elements can lead to excessive current flow through the switching element. Owing to the fact that such switching elements are designed for specific maximum current flows, the excessive current flow caused by a short circuit condition is likely to damage the switching element.
In a typical motor control circuit, the power source is connected through a transistor switching element to one side of the motor, with the other side of the motor being connected to the opposite pole of the power source. A flyback diode is typically connected across the combined field and armature of the motor to form a circulating current path for motor current during the period of time that the switching element is turned "off". The flyback diode is subjected to particularly difficult operating conditions, and occasionally fails as a result of being overstressed during operation. A shorted flyback diode causes excessive current to flow from the power source, through the switching element and the shorted diode, back to the opposite pole of the power source. Such extreme magnitudes of current flow are known to quickly destroy the solid-state switching element.
It is desirable to sense the presence of such a short circuit condition and exercise appropriate control strategies in the event such a short circuit condition is detected. It is known, for example, to sense a short circuit condition by monitoring the magnitude of current flowing through a shunt in series with the motor in a meter control circuit, and to interrupt the flow of current in the event it becomes excessive. However, in some situations it is not desirable to interpose a current shunt in the motor control circuit, and in any event, a shunt in series with the motor cannot detect a shorted flyback diode. Likewise, it may be undesirable to shut down the motor control circuit permanently as it is possible that the short circuit condition is only temporary and that the motor control circuit can resume proper operation after a temporary interruption.
The present invention is directed to overcoming one or more of the problems as set forth above.